2 Answers
2

These are almost certainly parasites called Scale insects which suck the juices out of the stem and leaves and cause the plant to lose vigor. In this case it is a Coccus. Fortunately, you have spotted them early, before your avocado has become infested with them, and they can easily be removed in one of the following ways:

Scrape them off with a cotton swab that you have dipped in rubbing alcohol;

Spray them with a home-made or insecticidal soap solution once every two or three weeks (see here); you will need to do this at least three times to prevent them from returning.

There is a useful article about these bugs and how to control them, here, and another, very detailed one, by the Utah State University Extension, here.

Scale insect attack can be a symptom of weaker than normal plant cells, such as produced by overfertilizing with nitrogen. So a longer term solution in this case is to reduce nitrogen fertilizer and/or add some potash to help grow stronger plant cells.

Ants help to spread the scale on the plant by protecting them from potential insect predators. The ants do so as the scale exudes sugary substance that ants like. In a sense the ants are said to be "farming the scale."